Indium tin oxide (ITO) has been generally used as a transparent conducting electrode material in display fields due to high electrical conductivity and optical transparency. However, their properties are steeply decreased with bending on flexible substrates, especially increment of a sheet resistance above 1,000 times. Besides, a difference in thermal expansion coefficients between the ITO electrode and the plastic substrate causes many problems in application to the flexible devices. Currently, conductive polymer, carbon nanotubes (CNTs), carbon nanofibers (CNFs), and the like have been actively investigated in order to replace the ITO used as conducting electrodes. CNTs, high conductivity and an excellent adhering property on plastic substrates as well as mechanically and thermally stable properties, have been in the spotlight as next generation electrode material. In spite of these advantages, the CNTs still remain solving problems such as a low input/output current and poor contact surface between CNTs as well as synthesis of aligned CNTs on a required location in application to electronic device.
Graphene, two-dimensional plane of carbon atoms, has outstanding electrical, mechanical, and thermal properties. Its conductivity is about ˜100 times faster than that of silicon at a room temperature and the current density is ˜100 times per unit area higher than that of copper. Besides, they possess twice or more thermal conductivity higher than that of diamond and ˜200 times mechanical strength with a high transparency compared to that of steel. Especially, hexagonal honeycomb structure of the graphene connected like a net can maintain the elasticity without noticeable signs of damage in electrical conductivity even for the unusual expanding or bending. Consequently, the unique properties of the graphene can bring replacement of ITO used as transparent conducting electrode and silicon mainly utilized as a semiconductor.
A scalable and economical route for generating the graphene without extreme degradations is required in order to apply the graphene having excellent properties to a flexible electronic device.
Generally, it has been known that the graphene is prepared by various methods such as mechanical exfoliation, epitaxial growth, thermal/gas expansion, chemical vapor deposition (CVD), graphene oxidation/reduction, graphite intercalation compound (GIC), and the like. Recently, the CVD and the graphene oxidation/reduction methods are actively researched so as to obtain high quality or large quantity of the graphene.
In mechanical exfoliation, a Scotch tape is attached to a graphite sample and then is detached from the graphite sample so as to obtain the graphene in the form of a sheet detached from graphite from the surface of the Scotch tape. In this case, the number of layers of the detached graphene sheet is not uniform, and its shape is a paper-torn shape and is not uniform. Furthermore, it is very difficult to obtain a large amount of scalable graphene sheets.
In the epitaxial growth, layers of the graphene are grown on a single crystalline silicon carbide (SiC) substrate. In thermal/gas expansion, a graphite oxide is heated at 1000° C. or higher so as to remove the graphite oxide and simultaneously to separate layers of graphite from each other, thereby producing the graphene. In the gaseous phase method, argon gas and ethanol aerosol are injected into a micro plasma reactor so as to form an argon plasma and to induce evaporation and decomposition of ethanol, and formation of the argon plasma is stopped so as to produce graphene in a solid state.
In CVD method, a catalyst metal is deposited on a substrate so as to form a thin metal layer, gas including carbon, argon, and hydrogen are flown to the metal layer at a high temperature of 800° C. or higher and then the thin metal layer is cooled down, thereby obtaining graphene formed on the metal layer. However, when a process of producing the graphene is performed at a high temperature of 700° C. or higher, the graphene may be damaged, and a process cost increases greatly.
FIGS. 1A and 1B are graphs showing the result of thermogravimetric analysis (TGA) of graphite, i.e., a variation of mass when graphite powder was heated. FIG. 1A is a graph showing the result of TGA of graphite when graphite powder was heated in the air including oxygen, and FIG. 1B is a graph showing the result of TGA of graphite at a nitrogen atmosphere in which oxygen is blocked.
As shown in FIGS. 1A and 1B, mass is decreased from a temperature of 700° C. or higher regardless of the case that oxygen is included (FIG. 1A) and the nitrogen atmosphere in which oxygen is blocked (FIG. 1B), due to oxidation of graphite and damage of a sp2 bond. However, since most existing graphene producing processes are performed at a high temperature of 700° C. or higher, the graphene may be damaged, a process procedure is complicated, and a process cost increases.
As well as the disadvantage of too high process temperature, in the CVD method, the graphene may be damaged during a catalyst removing process, and scalable and economical graphene cannot be generated. In the graphene oxidation-reduction method, oxygen atoms are not fully removed during a reduction process.
The method of obtaining the graphene by oxidizing, dispersing and reducing graphite is a widely-used method. However, oxygen atoms are not fully removed during a reduction process.
FIGS. 2 through 4 show properties of the graphene produced by using the conventional oxidation-reduction method. FIGS. 2A through 2C show the result of X-ray photoelectron spectroscopy (XPS) of the graphene produced by using the conventional oxidation-reduction method. FIG. 2A shows the result of XPS of oxidation-reduction graphene before oxidized graphenes are reduced, FIG. 2B shows the result of XPS of oxidation-reduction graphene after reduction is performed, and FIG. 2C shows the result of XPS of oxidation-reduction graphene after thermal treatment is performed after reduction. As shown in FIGS. 2A through 2C, even when oxidized graphene are reduced and then thermally treated, considerable oxygen atoms (20% or more) are not removed and remain (Nature Nanotechnology 3, 270 (2008)).
FIG. 3 is a graph showing the result of Raman spectroscopy of graphene produced by using the conventional oxidation-reduction method. In FIG. 3, graph indicated by reference numeral 210 shows the result of Raman spectroscopy of one layer of graphene produced by using the conventional oxidation-reduction method, and graph indicated by reference numeral 220 shows the result of Raman spectroscopy of 10 layers of graphene produced by using the conventional oxidation-reduction method.
As shown in FIG. 3, in graphenes produced by using the conventional oxidation-reduction method, a D-peak is higher than a G-peak. A D-peak in the vicinity of 1,350 cm−1 of Raman shift is increased when an impurity, such as oxygen, is included in the graphene (Nature Nanotechnology 3, 270 (2008)).
FIG. 4 is a graph showing the relationship between a sheet resistance and transmittance of the graphene. The less oxygen atoms removed, the higher transmittance of the graphene; however, a sheet resistance of the graphene is also increased. Since transmittance and sheet resistance are significant in using the graphene to form a transparent conducting layer, there may be a problem if oxygen atoms are not removed.
Thus, when the graphene is produced by using the conventional graphene oxidation-reduction method, oxygen atoms cannot be fully removed from the graphene and thus high quality of the graphene cannot be produced.
In the current GIC method, metal is inserted in graphite intercalation. An original graphite intercalation interval is 3.35 Å. However, when alkaline metals or alkaline earth metal ions are inserted in graphite intercalation, its intercalation interval is increased. In this case, the intercalation interval is further increased as an atomic radius of inserted ions of an element disposed on the lower part of the periodic table is increased.
However, according to the related art, metal is directly used to insert alkaline metal ions or alkaline earth metal ions are inserted in graphite intercalation, metal itself or metal is molten in a proper organic solvent and reacts with graphite, thereby producing a GIC. Since alkaline metals and alkaline earth metals as elements that belong to Groups 1 and 2 of the periodic table have very large reactivity, a process cannot be performed at an oxygen atmosphere, and they have very large explosiveness and thus it is difficult and dangerous to handle them.
In the production of a GIC, when molecules, such as tetrahydrofuran (THF), as well alkaline metal ions or alkaline earth metal ions are inserted in graphite intercalation (this is referred to as cointercalation), an interval of graphite intercalation is further increased, and dispersion into the graphene may be more easily performed. Table 1 shows an increase in an interval of graphite intercalation when metal ions are inserted in graphite intercalation and a further increase in the interval of the graphite intercalation when THF is added to metal ions.
TABLE 1Graphite IntercalationInserted MaterialInterval (Å)When there is no inserted material,3.35(pure graphite)Lithium3.74Potassium5.41Lithium + THF (Cointercalation)7.24~12.45Potassium + THF (Cointercalation)7.15~12.27